1. in Windows Media Player (V11 in my case), do Tools/Options/Library/Configure Sharing
2. this should show "unknown device" (if it has already seen your n900) - if so, goto (3). if not:
2.1 select "settings" and then tick "allow new devices and computers automatically"
2.2 click OK and go back to previous screen. It may now show the device (if not, exit/reenter the dialogs)
2.3 once the device is shown, go back and uncheck "allow new devices" (its insecure)
3. In the settings, select types of mdia to share (you can choose pictures, audio, video)
4. n900 should then show you a set of folders, one for each type of shared media content, when you click your XP box's name in MPlayer's home screen

Hope that helps